Background: Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is routinely used for analysis of immunosuppressive drugs. This study investigated whether replacing analog internal standards (ANISs) with isotopically labeled internal standards (ILISs) has an impact on the performance of a LC-MS/MS method for the quantification of tacrolimus (TAC), sirolimus (SIR), ciclosporin A (CsA) and everolimus (EVE) in whole blood.
Methods: Following hemolysis, protein precipitation, and extraction with either ANISs (ascomycin, desmethoxy-rapamycin, CsD), or ILISs (TAC-13C,D2; SIR-13C,D3; CsA-D12; EVE-D4), samples were centrifuged and injected into a LC-MS/MS device equipped with a C18 reversed phase column. The effect of the two ISs on the linearity, precision, accuracy, trueness, matrix effects, and carryover was investigated by using the same patient-, proficiency testing-, and quality control samples. Statistical analysis of agreement between results includes a standard random effects model and Passing-Bablok regression.
Results: Within-day imprecision was <10%, between-day <8%, and trueness 91%-110% for all the analytes with both ISs. No carryover or matrix effects were observed. The median accuracy was -2.1% for CsA, 9.1% for EVE, 12.2% for SIR, and -1.2% for TAC with the ILISs; and -2% for CsA, 9.8% for EVE, 11.4% for SIR, and 0.2% for TAC with the ANISs. Results of patient and proficiency testing samples were not statistically different.
Conclusions: Although ILISs are generally considered superior to ANISs, they may not be always essential. When optimizing a LC-MS/MS method other factors must be also considered.